The development of food products having improved texture and structural properties and which do not exhibit storage stability problems is of considerable commercial interest. In the case of ice cream confections, for example, there is a particular need to devise improved methods of combating textural deterioration resulting from the formation of ice crystals caused by cyclic changes in the freezing temperature (temperature abuse). Various approaches to overcoming these problems have been described, typically involving the addition of conventional stabilising, thickening and/or structuring additives to the food product during preparation. Polysaccharides such as locust bean gum, guar gum and carrageenan, for example, are commonly added to mask the perception of ice crystals and to improve the texture of ice cream products. Other stabilising agents which are used in food products include homopolysaccharides produced extracellularly by certain bacteria, such as dextran and fructans. Spray-dried dextran-milk powder is available commercially from Quest International under the trade name “Enrich”.
Dextrans are homopolysaccharides comprising glucose residues which are produced by the action of the enzyme dextransucrase on sucrose, these enzymes being produced primarily extracellularly by various lactic acid bacteria including strains from the genera Streptococcus, Leuconostoc and Lactobacillus. 
Controlled fermentation of a whey-based medium containing sucrose with the dextran-producing bacterium Leuconostoc mesenteroides ATCC 14935 to produce a thickening agent for use in foods is described in U.S. Pat. No. 4,444,793. The desired dextran content is produced by prolonged fermentation during which excessive acidification is avoided either by inclusion of buffer salts in the growth medium or by titration with base during fermentation.
U.S. Pat. No. 4,877,634 describes the preparation of a spray-dried dextran and levan (polyfructose) containing composition which gives high viscosity when rehydrated, by fermenting an aqueous growth medium comprising sucrose with Leuconostoc dextranicum NRRL-B-18132. The use of this composition to improve the thickness, stability or texture of food products, including milk drinks, salad dressings, ice cream and frozen yoghurt is also described. The production of dextran from another Leuconostoc dextranicum strain (NRRL-B-18242) and its use, preferably in the form of a dried rehydratable powder, as a texture modifying or improving additive in a similar range of foods is described in U.S. Pat. No. 5,223,431.
According to the taxonomy established by Garvie et al (International Journal of Systematic Bacteriology, 118-119,1983), bacteria of the genus Leuconostoc, previously classified as Leuconostoc mesenteroides, Leuconostoc dextranicum and Leuconostoc cremoris, are subspecies of Leuconostoc mesenteroides. In addition to the dextran producing subspecies of Leuconostoc mesenteroides mentioned above, two further dextran producing strains of use in texturing food products have been described in the literature, namely Leuconostoc mesenteroides ssp. cremoris strains CNCM I-1692 and CNCM-I-1693 (see U.S. Pat. No. 6,004,800).
As polysaccharides such as dextran are prepared by anaerobic fermentation of polysaccharide producing strains of lactic acid bacteria, their production is necessarily accompanied by the production of acid (and hence a lowering of pH) as a by-product of anaerobic metabolism. This has generally been regarded as limiting the applicability of methods for producing dextran by microbial fermentation in situ in the food product, itself particularly in the case of pH sensitive foods such as those containing milk proteins where lowering the pH can have marked effects on product texture, processability, stability and flavour. Where the product is, for example, a yoghurt or yoghurt-ice, the preparation of which normally involves acidification, this may present no problem but more generally the effects of a substantial drop in pH, especially in combination with high temperature when pasteurisation is necessary after the fermentation step, are likely to be highly detrimental to the quality of the product.
In those instances discussed above where dextran has been proposed as a suitable stabilising and texturing aid for ice cream products, for example, it is notable that it is always added in the form of a pre-prepared, pH adjusted additive. U.S. Pat. No. 6,004,800 describes the formation of an ice cream where dextran is produced during the manufacturing process (by a method which would not be subject to the problems associated with microbial metabolism) but again this involves the addition of an additive (here the purified enzyme, active dextransucrase) which has undesirable implications in terms of cost and consumer acceptability.
By contrast, instances of the production of polysaccharide texturing/structuring agents by fermentation in situ in yoghurt manufacture have been reported in the literature; here, where acid production is both a normal and desired part of the production process, the drop in pH accompanying fermentation does not present problems.
U.S. Pat. No. 6,004,800 (mentioned above) describes the preparation of a yoghurt of acceptable texture and taste by fermentation of dextran producing Leuconostoc mesenteroides ssp. cremoris CNCM I-1692 (together with non-thickening strains of Streptococcus thermophilus and Lactobacillus bulgaricus) in the presence of a milk-sucrose medium until the pH reaches a value of 4.5. This reference also discloses yoghurt formation using the enzyme dextransucrase, produced by Leuconostoc mesenteroides ssp cremoris, in the presence of sucrose to produce dextran in situ. In another example U.S. Pat. No. 5,308,628 describes the preparation of a thickener-free, yoghurt-based dairy dessert by culturing Lactobacillus acidophilus, Lactobacillus bifidus and/or Streptococcus thermophilus with various milk constituents to a pH between pH 4.3 and pH 5.5, so that the viscosity of the product is increased to an acceptable level.
Not only is the use of additives to improve the structure and texture of ice creams disadvantageous in economic terms but it is also undesirable in the light of increasing consumer desire for additive-free food products. There therefore remains a clear need for the development of improved methods for preparing ice cream confections and other milk containing products having improved structure and stability properties whilst avoiding the use of additives.